Solid tumors have long been known constantly to shed many biomolecules
such as DNA, RNA and proteins into the blood and other body fluids. These
biomolecules can circulate in the blood free of cells or by binding to proteins or lipids.
Circulating tumor DNA (ctDNA) is tumor-derived cell-free DNA (cfDNA) and is often
confused with non-tumor derived cfDNA. Recent advances in laboratory techniques
enable better capture and analysis of trace amounts of circulating materials. Liquid
biopsy is a minimally invasive method and allows analyses of circulating tumor cells
released from peripheral tumors and/or metastatic tumors and nucleic acids in the cellfree
circulation, in particular ctDNA, microRNA and extracellular RNA. The fact that
ctDNA completely reflects the tumor genome has made it a powerful clinical and
research tool in liquid biopsy, and a number of studies have been conducted on the
diagnostic, predictive and / or prognostic use of ctDNA in cancer over the last few
years. There are studies confirming the clinical validity of ctDNA for detecting tumor
heterogeneity and resistance mutation, identifying candidates for targeted therapies,
disease monitoring and therapeutic response assessment, early detection of recurrence,
monitoring tumor burden, and risk classification. In this chapter, we have summarized
the roles of ctDNA in clinical practice for diagnosis, treatment choices and responses to
therapy in various solid cancers.
Keywords: Breast cancer, Cell-free DNA, Circulating tumor cells, Circulating
tumor DNA, Colorectal cancer, Head and neck cancer squamous cell carcinoma,
Liquid biopsy, Non-small cell lung cancer, Prostate cancer gastrointestinal
stromal tumor, Solid tumors.